The progressive reduction of the dimensions in photolithographic manufacturing processes, in particular for integrated semiconductor circuits, is approaching its physical limits whenever the structures to be imaged are of the order of the light wavelength used and the resolution is confined by diffraction effects.
Therefore, corpuscular or X-rays, which correspond to much shorter wavelengths, have been considered for submicron lithography. Of these alternative methods, electron beam lithography with controlled beam deflection has been developed furthest. But with the latter type of method the time required for writing increases considerably as the structures are becoming smaller. This disadvantage is eliminated by projection methods, wherein masks are imaged, as, for example, in the case of the electron beam projection printing method according to the German Offenlegungsschrift No. OS 27 39 502. However, the resolution of all electron beam methods is generally limited by the so-called proximity effect which by scattering the electrons in the photoresist and the substrate prevents the imaging of very fine pattern elements. Although methods are known for compensating for the proximity effect in electron beam lithography [cf. European Patent Application No. 80103966.0 and the article by M. Parikh in J. Vac. Sci. Techn., Vol. 15, No. 3, (1978), p. 931], the expense involved is relatively great.
With lithographical methods using ion beams or X-rays, the proximity effect is not encountered. Although the minor scattering of these methods permits a high resolution, it is more difficult to align mask and substrate prior to exposure by means of the beams used for exposure. If the ions proper were to be used for alignment (also referred to as registration) they would have to penetrate the photoresist to be scattered at suitably placed registration marks. The scattered ions on their part would have to have sufficient energy to penetrate the photoresist a second time and to exit to the outside, in order to form a detectable signal for registration purposes.
In such circumstances registration signals that can be evaluated are obtained only if the energy chosen for the incident ions is very high or if the photoresist on top of the registration marks is removed. However, one method leads to lattice defects in semiconductors, whereas the other necessitates an additional exposure and development step.
Similar problems resulting from the low radiation density and the limited range also make it impossible with X-ray lithography to receive a registration signal in acceptable times.
Therefore, registration using optical methods has been suggested for the previously proposed ion beam and X-ray lithography systems; examples are described in the articles by B. Fay et al., "Optical alignment system for submicron X-ray lithography" in J. Vac. Sci. Tech., 16 (6), November/December 1979, p. 1954, and R. L. Selinger et al., "Ion beams promise practical systems for submicrometer wafer lithography" in Electronics, Mar. 27, 1980, page 142. However, the use of different types of beams and different beam paths for exposure on the one hand and alignment on the other gives rise to the problem of having to adapt both beam paths to each other and of keeping them constant over longer periods of time. Difficulties may occur in particular when a wafer is successively exposed in several devices of the same type.